About 90 kilometres from the sea, above London, the river begins to show the tide caused by the North Sea. It is said that London was made capital of Roman Britain at the spot where the tides reached in 43 AD, but different things have pushed this spot farther up the river in the over 2000 years since then. At London, the water is slightly salty with sea salt.

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Like the Celts who lived in the area, the Romans called the river Thamesis. The Thames was an important way to go between London and Westminster in the 16th and 17th centuries. The guild of watermen took Londoners back and forth by ferry. One of them, John Taylor, the Water Poet (1580—1653), described the river in a poem.

In the 17th and 18th century, at a time some call the "Little Ice Age", the Thames often froze over in the winter. This led to the first "Frost Fair" in 1607, with a tent city set up on the river with lots of odd amusements, like ice bowling. The last time the river froze over was in 1814. The building of a new London Bridge in 1825 may have helped stop it from freezing: the new bridge had fewer pillars than the old, allowing the river to flow more easily, and stopping it from flowing slowly enough to freeze in cold winters.

By the 18th century, the Thames was one of the world's busiest waterways, as London became the centre of the very big British Empire. During this time one of the worst river disasters in England took place on 3 September 1878 on the Thames, when the crowded pleasure boat Princess Alice crashed into the Bywell Castle killing over 640 people.

The coming of rail transport and road transport, and the decline of the Empire in the years following 1914, have made the river less important than it was. London itself is no longer used much as a port, and the "Port of London" has moved down the river to Tilbury. The Thames has been greatly cleaned up, and life has returned to its dead waters.

In the early 1980s, the Thames Barrier was opened to control flooding. It is used many times a year to stop water damage to London's low lying areas up the river.